The purpose of this Core is to provide a centralized facility to rapidly evaluate the neutralizing antibody levels in sera or plasma specimens from Projects 1, 2, 3, and 4 of the program proposal. Neutralization assays will be performed in T-lymphocytic cell lines, as well as peripheral blood mononuclear cell cultures. The extent of type-specific, heterotypic or cross- clade neutralization will be determined using appropriate viral targets. T-cell line adapted laboratory strains of HIV (LAI, MN, SF), as well as T-cell tropic clinical isolates (X-tropic SI variants), and non-syncytium inducing (NSI) R-tropic viruses belonging to different Clades will be used in these studies. The core laboratory will also generate infectious molecular clones of chimeric HIV which contain PCR-amplified envelope gene sequences that correspond to envelope sequences represented in the recombinant PolyEnv1 vaccine used in Project 4. The centralization of these activities will consolidate the use of biohazardous materials to one BSL-3 facility. Furthermore, it will minimize variations in the results of neutralization assays performed for each of the four projects.